/*
 * Copyright 2002-2019 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.web.servlet;

import java.io.IOException;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.List;
import java.util.Locale;
import java.util.Map;
import java.util.Properties;
import java.util.Set;
import java.util.stream.Collectors;

import javax.servlet.DispatcherType;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.springframework.beans.factory.BeanFactoryUtils;
import org.springframework.beans.factory.BeanInitializationException;
import org.springframework.beans.factory.NoSuchBeanDefinitionException;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.context.i18n.LocaleContext;
import org.springframework.core.annotation.AnnotationAwareOrderComparator;
import org.springframework.core.io.ClassPathResource;
import org.springframework.core.io.support.PropertiesLoaderUtils;
import org.springframework.core.log.LogFormatUtils;
import org.springframework.http.server.ServletServerHttpRequest;
import org.springframework.lang.Nullable;
import org.springframework.ui.context.ThemeSource;
import org.springframework.util.ClassUtils;
import org.springframework.util.StringUtils;
import org.springframework.web.context.WebApplicationContext;
import org.springframework.web.context.request.ServletWebRequest;
import org.springframework.web.context.request.async.WebAsyncManager;
import org.springframework.web.context.request.async.WebAsyncUtils;
import org.springframework.web.multipart.MultipartException;
import org.springframework.web.multipart.MultipartHttpServletRequest;
import org.springframework.web.multipart.MultipartResolver;
import org.springframework.web.util.NestedServletException;
import org.springframework.web.util.WebUtils;

/**
 * Central dispatcher for HTTP request handlers/controllers, e.g. for web UI controllers
 * or HTTP-based remote service exporters. Dispatches to registered handlers for processing
 * a web request, providing convenient mapping and exception handling facilities.
 *
 * <p>This servlet is very flexible: It can be used with just about any workflow, with the
 * installation of the appropriate adapter classes. It offers the following functionality
 * that distinguishes it from other request-driven web MVC frameworks:
 *
 * <ul>
 * <li>It is based around a JavaBeans configuration mechanism.
 *
 * <li>It can use any {@link HandlerMapping} implementation - pre-built or provided as part
 * of an application - to control the routing of requests to handler objects. Default is
 * {@link org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping} and
 * {@link org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping}.
 * HandlerMapping objects can be defined as beans in the servlet's application context,
 * implementing the HandlerMapping interface, overriding the default HandlerMapping if
 * present. HandlerMappings can be given any bean name (they are tested by type).
 *
 * <li>It can use any {@link HandlerAdapter}; this allows for using any handler interface.
 * Default adapters are {@link org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter},
 * {@link org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter}, for Spring's
 * {@link org.springframework.web.HttpRequestHandler} and
 * {@link org.springframework.web.servlet.mvc.Controller} interfaces, respectively. A default
 * {@link org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter}
 * will be registered as well. HandlerAdapter objects can be added as beans in the
 * application context, overriding the default HandlerAdapters. Like HandlerMappings,
 * HandlerAdapters can be given any bean name (they are tested by type).
 *
 * <li>The dispatcher's exception resolution strategy can be specified via a
 * {@link HandlerExceptionResolver}, for example mapping certain exceptions to error pages.
 * Default are
 * {@link org.springframework.web.servlet.mvc.method.annotation.ExceptionHandlerExceptionResolver},
 * {@link org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver}, and
 * {@link org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver}.
 * These HandlerExceptionResolvers can be overridden through the application context.
 * HandlerExceptionResolver can be given any bean name (they are tested by type).
 *
 * <li>Its view resolution strategy can be specified via a {@link ViewResolver}
 * implementation, resolving symbolic view names into View objects. Default is
 * {@link org.springframework.web.servlet.view.InternalResourceViewResolver}.
 * ViewResolver objects can be added as beans in the application context, overriding the
 * default ViewResolver. ViewResolvers can be given any bean name (they are tested by type).
 *
 * <li>If a {@link View} or view name is not supplied by the user, then the configured
 * {@link RequestToViewNameTranslator} will translate the current request into a view name.
 * The corresponding bean name is "viewNameTranslator"; the default is
 * {@link org.springframework.web.servlet.view.DefaultRequestToViewNameTranslator}.
 *
 * <li>The dispatcher's strategy for resolving multipart requests is determined by a
 * {@link org.springframework.web.multipart.MultipartResolver} implementation.
 * Implementations for Apache Commons FileUpload and Servlet 3 are included; the typical
 * choice is {@link org.springframework.web.multipart.commons.CommonsMultipartResolver}.
 * The MultipartResolver bean name is "multipartResolver"; default is none.
 *
 * <li>Its locale resolution strategy is determined by a {@link LocaleResolver}.
 * Out-of-the-box implementations work via HTTP accept header, cookie, or session.
 * The LocaleResolver bean name is "localeResolver"; default is
 * {@link org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolver}.
 *
 * <li>Its theme resolution strategy is determined by a {@link ThemeResolver}.
 * Implementations for a fixed theme and for cookie and session storage are included.
 * The ThemeResolver bean name is "themeResolver"; default is
 * {@link org.springframework.web.servlet.theme.FixedThemeResolver}.
 * </ul>
 *
 * <p><b>NOTE: The {@code @RequestMapping} annotation will only be processed if a
 * corresponding {@code HandlerMapping} (for type-level annotations) and/or
 * {@code HandlerAdapter} (for method-level annotations) is present in the dispatcher.</b>
 * This is the case by default. However, if you are defining custom {@code HandlerMappings}
 * or {@code HandlerAdapters}, then you need to make sure that a corresponding custom
 * {@code RequestMappingHandlerMapping} and/or {@code RequestMappingHandlerAdapter}
 * is defined as well - provided that you intend to use {@code @RequestMapping}.
 *
 * <p><b>A web application can define any number of DispatcherServlets.</b>
 * Each servlet will operate in its own namespace, loading its own application context
 * with mappings, handlers, etc. Only the root application context as loaded by
 * {@link org.springframework.web.context.ContextLoaderListener}, if any, will be shared.
 *
 * <p>As of Spring 3.1, {@code DispatcherServlet} may now be injected with a web
 * application context, rather than creating its own internally. This is useful in Servlet
 * 3.0+ environments, which support programmatic registration of servlet instances.
 * See the {@link #DispatcherServlet(WebApplicationContext)} javadoc for details.
 *
 * DispatcherServlet 前端控制器对象，也是一个 servlet
 *       HttpServlet
 *       	HttpServletBean
 *       		FrameworkServlet
 *       		    DispatcherServlet
 *
 * 我们分析一条 request 是如何执行的，然后找到对应的 controller
 * 1、请求过来之后，调用 org.springframework.web.servlet.DispatcherServlet#doService(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse)
 * 2、
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @author Rob Harrop
 * @author Chris Beams
 * @author Rossen Stoyanchev
 * @see org.springframework.web.HttpRequestHandler
 * @see org.springframework.web.servlet.mvc.Controller
 * @see org.springframework.web.context.ContextLoaderListener
 */
@SuppressWarnings("serial")
public class DispatcherServlet extends FrameworkServlet {
    
    /** Well-known name for the MultipartResolver object in the bean factory for this namespace. */
    /**
     * 文件上传下载组件的名称
     */
    public static final String MULTIPART_RESOLVER_BEAN_NAME = "multipartResolver";
    
    /** Well-known name for the LocaleResolver object in the bean factory for this namespace. */
    /**
     * 国际化支持组件的名称
     */
    public static final String LOCALE_RESOLVER_BEAN_NAME = "localeResolver";
    
    /** Well-known name for the ThemeResolver object in the bean factory for this namespace. */
    public static final String THEME_RESOLVER_BEAN_NAME = "themeResolver";
    
    /**
     * Well-known name for the HandlerMapping object in the bean factory for this namespace.
     * Only used when "detectAllHandlerMappings" is turned off.
     * @see #setDetectAllHandlerMappings
     */
    public static final String HANDLER_MAPPING_BEAN_NAME = "handlerMapping";
    
    /**
     * Well-known name for the HandlerAdapter object in the bean factory for this namespace.
     * Only used when "detectAllHandlerAdapters" is turned off.
     * @see #setDetectAllHandlerAdapters
     */
    public static final String HANDLER_ADAPTER_BEAN_NAME = "handlerAdapter";
    
    /**
     * Well-known name for the HandlerExceptionResolver object in the bean factory for this namespace.
     * Only used when "detectAllHandlerExceptionResolvers" is turned off.
     * @see #setDetectAllHandlerExceptionResolvers
     */
    public static final String HANDLER_EXCEPTION_RESOLVER_BEAN_NAME = "handlerExceptionResolver";
    
    /**
     * Well-known name for the RequestToViewNameTranslator object in the bean factory for this namespace.
     */
    public static final String REQUEST_TO_VIEW_NAME_TRANSLATOR_BEAN_NAME = "viewNameTranslator";
    
    /**
     * Well-known name for the ViewResolver object in the bean factory for this namespace.
     * Only used when "detectAllViewResolvers" is turned off.
     * @see #setDetectAllViewResolvers
     */
    public static final String VIEW_RESOLVER_BEAN_NAME = "viewResolver";
    
    /**
     * Well-known name for the FlashMapManager object in the bean factory for this namespace.
     */
    public static final String FLASH_MAP_MANAGER_BEAN_NAME = "flashMapManager";
    
    /**
     * Request attribute to hold the current web application context.
     * Otherwise only the global web app context is obtainable by tags etc.
     * @see org.springframework.web.servlet.support.RequestContextUtils#findWebApplicationContext
     */
    public static final String WEB_APPLICATION_CONTEXT_ATTRIBUTE = DispatcherServlet.class.getName() + ".CONTEXT";
    
    /**
     * Request attribute to hold the current LocaleResolver, retrievable by views.
     * @see org.springframework.web.servlet.support.RequestContextUtils#getLocaleResolver
     */
    public static final String LOCALE_RESOLVER_ATTRIBUTE = DispatcherServlet.class.getName() + ".LOCALE_RESOLVER";
    
    /**
     * Request attribute to hold the current ThemeResolver, retrievable by views.
     * @see org.springframework.web.servlet.support.RequestContextUtils#getThemeResolver
     */
    public static final String THEME_RESOLVER_ATTRIBUTE = DispatcherServlet.class.getName() + ".THEME_RESOLVER";
    
    /**
     * Request attribute to hold the current ThemeSource, retrievable by views.
     * @see org.springframework.web.servlet.support.RequestContextUtils#getThemeSource
     */
    public static final String THEME_SOURCE_ATTRIBUTE = DispatcherServlet.class.getName() + ".THEME_SOURCE";
    
    /**
     * Name of request attribute that holds a read-only {@code Map<String,?>}
     * with "input" flash attributes saved by a previous request, if any.
     * @see org.springframework.web.servlet.support.RequestContextUtils#getInputFlashMap(HttpServletRequest)
     */
    public static final String INPUT_FLASH_MAP_ATTRIBUTE = DispatcherServlet.class.getName() + ".INPUT_FLASH_MAP";
    
    /**
     * Name of request attribute that holds the "output" {@link FlashMap} with
     * attributes to save for a subsequent request.
     * @see org.springframework.web.servlet.support.RequestContextUtils#getOutputFlashMap(HttpServletRequest)
     */
    public static final String OUTPUT_FLASH_MAP_ATTRIBUTE = DispatcherServlet.class.getName() + ".OUTPUT_FLASH_MAP";
    
    /**
     * Name of request attribute that holds the {@link FlashMapManager}.
     * @see org.springframework.web.servlet.support.RequestContextUtils#getFlashMapManager(HttpServletRequest)
     */
    public static final String FLASH_MAP_MANAGER_ATTRIBUTE = DispatcherServlet.class.getName() + ".FLASH_MAP_MANAGER";
    
    /**
     * Name of request attribute that exposes an Exception resolved with a
     * {@link HandlerExceptionResolver} but where no view was rendered
     * (e.g. setting the status code).
     */
    public static final String EXCEPTION_ATTRIBUTE = DispatcherServlet.class.getName() + ".EXCEPTION";
    
    /** Log category to use when no mapped handler is found for a request. */
    public static final String PAGE_NOT_FOUND_LOG_CATEGORY = "org.springframework.web.servlet.PageNotFound";
    
    /**
     * Name of the class path resource (relative to the DispatcherServlet class)
     * that defines DispatcherServlet's default strategy names.
     */
    private static final String DEFAULT_STRATEGIES_PATH = "DispatcherServlet.properties";
    
    /**
     * Common prefix that DispatcherServlet's default strategy attributes start with.
     */
    private static final String DEFAULT_STRATEGIES_PREFIX = "org.springframework.web.servlet";
    
    /** Additional logger to use when no mapped handler is found for a request. */
    protected static final Log pageNotFoundLogger = LogFactory.getLog(PAGE_NOT_FOUND_LOG_CATEGORY);
    
    private static final Properties defaultStrategies;
    
    static {
        /**
         * 用于加载我们的 DispatcherServlet.properties 文件内容到 properties 属性对象中
         */
        // Load default strategy implementations from properties file.
        // This is currently strictly internal and not meant to be customized
        // by application developers.
        try {
            ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, DispatcherServlet.class);
            defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
        } catch (IOException ex) {
            throw new IllegalStateException("Could not load '" + DEFAULT_STRATEGIES_PATH + "': " + ex.getMessage());
        }
    }
    
    /** Detect all HandlerMappings or just expect "handlerMapping" bean?. */
    /**
     * 是否允许探测所有的 HandlerMappings ，默认是 true
     */
    private boolean detectAllHandlerMappings = true;
    
    /** Detect all HandlerAdapters or just expect "handlerAdapter" bean?. */
    /**
     * 是否允许探测所有的处理适配器
     */
    private boolean detectAllHandlerAdapters = true;
    
    /** Detect all HandlerExceptionResolvers or just expect "handlerExceptionResolver" bean?. */
    /**
     * 是否允许探测所有的异常解析器
     */
    private boolean detectAllHandlerExceptionResolvers = true;
    
    /** Detect all ViewResolvers or just expect "viewResolver" bean?. */
    /**
     * 是否允许探测所有的视图解析器
     */
    private boolean detectAllViewResolvers = true;
    
    /** Throw a NoHandlerFoundException if no Handler was found to process this request? *.*/
    private boolean throwExceptionIfNoHandlerFound = false;
    
    /** Perform cleanup of request attributes after include request?. */
    private boolean cleanupAfterInclude = true;
    
    /** MultipartResolver used by this servlet. */
    /**
     * spring 本身是不提供文件上传下载的，所以通过 MultipartResolver 接口来提供
     * 常用子类：CommonsMultipartResolver
     */
    @Nullable
    private MultipartResolver multipartResolver;
    
    /** LocaleResolver used by this servlet. */
    /**
     * 国际化支持
     */
    @Nullable
    private LocaleResolver localeResolver;
    
    /** ThemeResolver used by this servlet. */
    @Nullable
    private ThemeResolver themeResolver;
    
    /** List of HandlerMappings used by this servlet. */
    /**
     * HandlerMapping 负责映射用户的 URI 和对应的处理类（controller）
     */
    @Nullable
    private List<HandlerMapping> handlerMappings;
    
    /** List of HandlerAdapters used by this servlet. */
    /**
     * 在HandlerMapping接口中只定义了根据一个URL必须返回一个由HandlerExecutionChain代表的处理链，我们可以在这个处理链中添加任意的HandlerAdapter实例来处理这个URL对应的请求。
     */
    @Nullable
    private List<HandlerAdapter> handlerAdapters;
    
    /** List of HandlerExceptionResolvers used by this servlet. */
    @Nullable
    private List<HandlerExceptionResolver> handlerExceptionResolvers;
    
    /** RequestToViewNameTranslator used by this servlet. */
    @Nullable
    private RequestToViewNameTranslator viewNameTranslator;
    
    /** FlashMapManager used by this servlet. */
    @Nullable
    private FlashMapManager flashMapManager;
    
    /** List of ViewResolvers used by this servlet. */
    @Nullable
    private List<ViewResolver> viewResolvers;
    
    
    /**
     * Create a new {@code DispatcherServlet} that will create its own internal web
     * application context based on defaults and values provided through servlet
     * init-params. Typically used in Servlet 2.5 or earlier environments, where the only
     * option for servlet registration is through {@code web.xml} which requires the use
     * of a no-arg constructor.
     * <p>Calling {@link #setContextConfigLocation} (init-param 'contextConfigLocation')
     * will dictate which XML files will be loaded by the
     * {@linkplain #DEFAULT_CONTEXT_CLASS default XmlWebApplicationContext}
     * <p>Calling {@link #setContextClass} (init-param 'contextClass') overrides the
     * default {@code XmlWebApplicationContext} and allows for specifying an alternative class,
     * such as {@code AnnotationConfigWebApplicationContext}.
     * <p>Calling {@link #setContextInitializerClasses} (init-param 'contextInitializerClasses')
     * indicates which {@code ApplicationContextInitializer} classes should be used to
     * further configure the internal application context prior to refresh().
     * @see #DispatcherServlet(WebApplicationContext)
     */
    public DispatcherServlet() {
        super();
        setDispatchOptionsRequest(true);
    }
    
    /**
     * Create a new {@code DispatcherServlet} with the given web application context. This
     * constructor is useful in Servlet 3.0+ environments where instance-based registration
     * of servlets is possible through the {@link ServletContext#addServlet} API.
     * <p>Using this constructor indicates that the following properties / init-params
     * will be ignored:
     * <ul>
     * <li>{@link #setContextClass(Class)} / 'contextClass'</li>
     * <li>{@link #setContextConfigLocation(String)} / 'contextConfigLocation'</li>
     * <li>{@link #setContextAttribute(String)} / 'contextAttribute'</li>
     * <li>{@link #setNamespace(String)} / 'namespace'</li>
     * </ul>
     * <p>The given web application context may or may not yet be {@linkplain
     * ConfigurableApplicationContext#refresh() refreshed}. If it has <strong>not</strong>
     * already been refreshed (the recommended approach), then the following will occur:
     * <ul>
     * <li>If the given context does not already have a {@linkplain
     * ConfigurableApplicationContext#setParent parent}, the root application context
     * will be set as the parent.</li>
     * <li>If the given context has not already been assigned an {@linkplain
     * ConfigurableApplicationContext#setId id}, one will be assigned to it</li>
     * <li>{@code ServletContext} and {@code ServletConfig} objects will be delegated to
     * the application context</li>
     * <li>{@link #postProcessWebApplicationContext} will be called</li>
     * <li>Any {@code ApplicationContextInitializer}s specified through the
     * "contextInitializerClasses" init-param or through the {@link
     * #setContextInitializers} property will be applied.</li>
     * <li>{@link ConfigurableApplicationContext#refresh refresh()} will be called if the
     * context implements {@link ConfigurableApplicationContext}</li>
     * </ul>
     * If the context has already been refreshed, none of the above will occur, under the
     * assumption that the user has performed these actions (or not) per their specific
     * needs.
     * <p>See {@link org.springframework.web.WebApplicationInitializer} for usage examples.
     * @param webApplicationContext the context to use
     * @see #initWebApplicationContext
     * @see #configureAndRefreshWebApplicationContext
     * @see org.springframework.web.WebApplicationInitializer
     */
    public DispatcherServlet(WebApplicationContext webApplicationContext) {
        /**
         * 将 webApplicationContext【子容器】 注入到 DispatcherServlet，
         */
        super(webApplicationContext);
        setDispatchOptionsRequest(true);
    }
    
    
    /**
     * Set whether to detect all HandlerMapping beans in this servlet's context. Otherwise,
     * just a single bean with name "handlerMapping" will be expected.
     * <p>Default is "true". Turn this off if you want this servlet to use a single
     * HandlerMapping, despite multiple HandlerMapping beans being defined in the context.
     */
    public void setDetectAllHandlerMappings(boolean detectAllHandlerMappings) {
        this.detectAllHandlerMappings = detectAllHandlerMappings;
    }
    
    /**
     * Set whether to detect all HandlerAdapter beans in this servlet's context. Otherwise,
     * just a single bean with name "handlerAdapter" will be expected.
     * <p>Default is "true". Turn this off if you want this servlet to use a single
     * HandlerAdapter, despite multiple HandlerAdapter beans being defined in the context.
     */
    public void setDetectAllHandlerAdapters(boolean detectAllHandlerAdapters) {
        this.detectAllHandlerAdapters = detectAllHandlerAdapters;
    }
    
    /**
     * Set whether to detect all HandlerExceptionResolver beans in this servlet's context. Otherwise,
     * just a single bean with name "handlerExceptionResolver" will be expected.
     * <p>Default is "true". Turn this off if you want this servlet to use a single
     * HandlerExceptionResolver, despite multiple HandlerExceptionResolver beans being defined in the context.
     */
    public void setDetectAllHandlerExceptionResolvers(boolean detectAllHandlerExceptionResolvers) {
        this.detectAllHandlerExceptionResolvers = detectAllHandlerExceptionResolvers;
    }
    
    /**
     * Set whether to detect all ViewResolver beans in this servlet's context. Otherwise,
     * just a single bean with name "viewResolver" will be expected.
     * <p>Default is "true". Turn this off if you want this servlet to use a single
     * ViewResolver, despite multiple ViewResolver beans being defined in the context.
     */
    public void setDetectAllViewResolvers(boolean detectAllViewResolvers) {
        this.detectAllViewResolvers = detectAllViewResolvers;
    }
    
    /**
     * Set whether to throw a NoHandlerFoundException when no Handler was found for this request.
     * This exception can then be caught with a HandlerExceptionResolver or an
     * {@code @ExceptionHandler} controller method.
     * <p>Note that if {@link org.springframework.web.servlet.resource.DefaultServletHttpRequestHandler}
     * is used, then requests will always be forwarded to the default servlet and a
     * NoHandlerFoundException would never be thrown in that case.
     * <p>Default is "false", meaning the DispatcherServlet sends a NOT_FOUND error through the
     * Servlet response.
     * @since 4.0
     */
    public void setThrowExceptionIfNoHandlerFound(boolean throwExceptionIfNoHandlerFound) {
        this.throwExceptionIfNoHandlerFound = throwExceptionIfNoHandlerFound;
    }
    
    /**
     * Set whether to perform cleanup of request attributes after an include request, that is,
     * whether to reset the original state of all request attributes after the DispatcherServlet
     * has processed within an include request. Otherwise, just the DispatcherServlet's own
     * request attributes will be reset, but not model attributes for JSPs or special attributes
     * set by views (for example, JSTL's).
     * <p>Default is "true", which is strongly recommended. Views should not rely on request attributes
     * having been set by (dynamic) includes. This allows JSP views rendered by an included controller
     * to use any model attributes, even with the same names as in the main JSP, without causing side
     * effects. Only turn this off for special needs, for example to deliberately allow main JSPs to
     * access attributes from JSP views rendered by an included controller.
     */
    public void setCleanupAfterInclude(boolean cleanupAfterInclude) {
        this.cleanupAfterInclude = cleanupAfterInclude;
    }
    
    
    /**
     * This implementation calls {@link #initStrategies}.
     *
     * 用于初始化我们 springmvc 的九大组件
     */
    @Override
    protected void onRefresh(ApplicationContext context) {
		/**
		 * 调用来源，org.springframework.web.servlet.FrameworkServlet#initWebApplicationContext()
		 * 传入的参数是子容器，子容器包含有【controller、view、HandlerMapping】
		 * 但是发现在源码实际获取 HandlerMapping 的时候，有一个注释说是包含祖先的 contexts
		 */
		initStrategies(context);
    }
    
    /**
     * 用于初始化我们 springmvc 的九大组件
     * Initialize the strategy objects that this servlet uses.
     * <p>May be overridden in subclasses in order to initialize further strategy objects.
     */
    protected void initStrategies(ApplicationContext context) {
		/**
		 * 初始化 web 上下文对象的 用于文件上传下载的解析器对象
		 * 处理 multipart-form-data 类型请求
		 */
		initMultipartResolver(context);
        // 初始化 国际化资源
        initLocaleResolver(context);
        // 初始化 主题解析器
        initThemeResolver(context);
		/**
		 * 初始化 handlermappings，handlerMapper 存储的是 uri 对应的 Controller 处理类。
		 * 当 DispatcherServlet 接受到客户端的请求后，SpringMVC 通过 uri 在 handlermappings 定位到 Controller 处理类
		 */
		initHandlerMappings(context);
		/**
		 * 初始化 handleradapters，我们拿到了 controller 就要调用它的对应的方法。adapter 负责方法调用。
		 */
		initHandlerAdapters(context);
        // 初始化 异常处理器
        initHandlerExceptionResolvers(context);
        //
        initRequestToViewNameTranslator(context);
		/**
		 * view 解析器
		 */
		initViewResolvers(context);
        initFlashMapManager(context);
    }
    
    /**
     * 初始化我们文件上传下载的组件
     * Initialize the MultipartResolver used by this class.
     * <p>If no bean is defined with the given name in the BeanFactory for this namespace,
     * no multipart handling is provided.
     */
    private void initMultipartResolver(ApplicationContext context) {
        try {
            /**
             * 去容器中获取 multipartResolver 名称组件，需要自己配置
             */
            this.multipartResolver = context.getBean(MULTIPART_RESOLVER_BEAN_NAME, MultipartResolver.class);
            if (logger.isTraceEnabled()) {
                logger.trace("Detected " + this.multipartResolver);
            } else if (logger.isDebugEnabled()) {
                logger.debug("Detected " + this.multipartResolver.getClass().getSimpleName());
            }
        } catch (NoSuchBeanDefinitionException ex) {
            // Default is no multipart resolver.
            this.multipartResolver = null;
            if (logger.isTraceEnabled()) {
                logger.trace("No MultipartResolver '" + MULTIPART_RESOLVER_BEAN_NAME + "' declared");
            }
        }
    }
    
    /**
     * 从容器中获取我们 处理语言支持组件
     * Initialize the LocaleResolver used by this class.
     * <p>If no bean is defined with the given name in the BeanFactory for this namespace,
     * we default to AcceptHeaderLocaleResolver.
     */
    private void initLocaleResolver(ApplicationContext context) {
        try {
            //从我们的web子容器中获取到我们处理国际化资源的组件(需要我们自己配置)
            this.localeResolver = context.getBean(LOCALE_RESOLVER_BEAN_NAME, LocaleResolver.class);
            if (logger.isTraceEnabled()) {
                logger.trace("Detected " + this.localeResolver);
            } else if (logger.isDebugEnabled()) {
                logger.debug("Detected " + this.localeResolver.getClass().getSimpleName());
            }
        } catch (NoSuchBeanDefinitionException ex) {
            // We need to use the default.
            // 用户自己没有配置，我们就使用系统默认的(AcceptHeaderLocaleResolver),
            // 默认配置 是通过SpringMvc配置在DispatcherServlet.properties属性文件中
            this.localeResolver = getDefaultStrategy(context, LocaleResolver.class);
            if (logger.isTraceEnabled()) {
                logger.trace("No LocaleResolver '" + LOCALE_RESOLVER_BEAN_NAME +
                        "': using default [" + this.localeResolver.getClass().getSimpleName() + "]");
            }
        }
    }
    
    /**
     * Initialize the ThemeResolver used by this class.
     * <p>If no bean is defined with the given name in the BeanFactory for this namespace,
     * we default to a FixedThemeResolver.
     */
    private void initThemeResolver(ApplicationContext context) {
        try {
            this.themeResolver = context.getBean(THEME_RESOLVER_BEAN_NAME, ThemeResolver.class);
            if (logger.isTraceEnabled()) {
                logger.trace("Detected " + this.themeResolver);
            } else if (logger.isDebugEnabled()) {
                logger.debug("Detected " + this.themeResolver.getClass().getSimpleName());
            }
        } catch (NoSuchBeanDefinitionException ex) {
            // We need to use the default.
            this.themeResolver = getDefaultStrategy(context, ThemeResolver.class);
            if (logger.isTraceEnabled()) {
                logger.trace("No ThemeResolver '" + THEME_RESOLVER_BEAN_NAME +
                        "': using default [" + this.themeResolver.getClass().getSimpleName() + "]");
            }
        }
    }
    
    /**
     * Initialize the HandlerMappings used by this class.
     * <p>If no HandlerMapping beans are defined in the BeanFactory for this namespace,
     * we default to BeanNameUrlHandlerMapping.
     */
    private void initHandlerMappings(ApplicationContext context) {
		this.handlerMappings = null;
	
		/**
		 * 是否允许查找所有的 handler。默认 true
		 */
		if (this.detectAllHandlerMappings) {
            // Find all HandlerMappings in the ApplicationContext, including ancestor contexts.
            /**
			 *******？？？？这里的源码注释说包含了祖先 context？是否是指的父级容器？
             * 从子容器【springmvc】获取所有实现了 HandlerMapping 接口的实现类。
			 *
			 * 1、没有加 @EnableWebMvc 注解的时候，且自己没有实现 HanlderMapping 的就会跳到最后使用默认的 HanlderMapping
			 * 2、如果我们在 springmvc 配置类加入了注解 @EnableWebMvc ，查看 @EnableWebMvc 这个注解包含 @Import(DelegatingWebMvcConfiguration.class)
			 * 导入了 DelegatingWebMvcConfiguration，而它的父类 WebMvcConfigurationSupport 是一个配置类, 配置了
			 * 1)requestMappingHandlerMapping  用于处理我们的 @RequestMapping
			 * 2)beanNameHandlerMapping(BeanNameUrlHandlerMapping) 基于 BeanName 映射请求
			 * ---
			 * 3)viewControllerHandlerMapping  用于处理我们的view 到页面的转发，不会经过我们的控制器对象
			 * 4)resourceHandlerMapping:资源映射
			 * 5)defaultServletHandlerMapping: 处理静态资源文件
			 ******
			 * 说回这里，我的 demo 使用了 @EnableWebMvc 注解，所以这里解析到了 3 个 HandlerMapping
			 * requestMappingHandlerMapping、beanNameHandlerMapping、routerFunctionMapping
			 * 这 3 个 bean 在 springmvc 容器初始化的时候被初始化。
			 * 我们以 requestMappingHandlerMapping 为例，这个类的父类 实现了 InitializingBean 接口。
			 * InitializingBean 接口为 bean 提供了属性初始化后的处理方法，它只包括 afterPropertiesSet 方法，
			 * 凡是继承该接口的类，在bean的属性初始化后都会执行该方法。
			 *
			 * 我们看 org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping#afterPropertiesSet()
			 * 调用到了 org.springframework.web.servlet.handler.AbstractHandlerMethodMapping#initHandlerMethods()
			 *
			 * 经过一系列的处理，我们得到了 uri 和 method 的对应关系
			 * 【至于方法的调用，在后面使用 调用链的 adapter 去调用】
			 *
			 * 在用户发起请求时，经过 doService，会有异步获取处理链的时候，会通过 uri 获取到的这里注册的方法。
			 *
			 * 参考：https://segmentfault.com/a/1190000012461362
			 *
			 * 在 springmvc 中对 controller 的定义方式有 2 类，3 种方式
			 *
			 * https://www.bilibili.com/video/BV1f7411K7UE?p=3 4 分 47s
			 *
             */
            Map<String, HandlerMapping> matchingBeans =
                    BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);
            // 找到了
            if (!matchingBeans.isEmpty()) {
                this.handlerMappings = new ArrayList<>(matchingBeans.values());
                // We keep HandlerMappings in sorted order.
                // 排序，spring 处理请求就是依据这个排序的结果进行，如果当前handlerMapping不可以处理则抛给下一个
                AnnotationAwareOrderComparator.sort(this.handlerMappings);
            }
        } else {
            try {
                HandlerMapping hm = context.getBean(HANDLER_MAPPING_BEAN_NAME, HandlerMapping.class);
                this.handlerMappings = Collections.singletonList(hm);
            } catch (NoSuchBeanDefinitionException ex) {
                // Ignore, we'll add a default HandlerMapping later.
            }
        }
        
        // Ensure we have at least one HandlerMapping, by registering
        // a default HandlerMapping if no other mappings are found.
		/**
		 * 确保有至少有一个 HandlerMapping，如果前面没有找到就使用默认的 HandlerMapping
		 */
        if (this.handlerMappings == null) {
			/**
			 * 没有使用 @EnableWebMvc 注解，进入默认方法，得到了 3 个默认的 HandlerMapping
			 * BeanNameUrlHandlerMapping（实现接口的 controller）
			 * RequestMappingHandlerMapping（基于注解的 controller）
			 * RouterFunctionMapping（Spring5.0 的 WebFlux 加入的，先忽略）
			 */
			this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);
            if (logger.isTraceEnabled()) {
                logger.trace("No HandlerMappings declared for servlet '" + getServletName() +
                        "': using default strategies from DispatcherServlet.properties");
            }
        }
    }
    
    /**
     * Initialize the HandlerAdapters used by this class.
     * <p>If no HandlerAdapter beans are defined in the BeanFactory for this namespace,
     * we default to SimpleControllerHandlerAdapter.
     */
    private void initHandlerAdapters(ApplicationContext context) {
		/**
		 * 这里和我们的 handlerMappings 一样的设计逻辑，
		 */
		this.handlerAdapters = null;
        
        if (this.detectAllHandlerAdapters) {
            // Find all HandlerAdapters in the ApplicationContext, including ancestor contexts.
			/**
			 * 找到所有实现了 HandlerAdapter 接口的实现类。
			 * 区分是否使用了注解 @EnableWebMvc
			 * 1、没使用，调用默认方法
			 * 2、使用了，注册了几个方法 RequestMappingHandlerAdapter、SimpleControllerHandlerAdapter、HttpRequestHandlerAdapter
			 * 同我们分析 handlerMapping 时一样，在 bean 属性赋值后调用 InitializingBean 接口 afterPropertiesSet 方法
			 *
			 * org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter#afterPropertiesSet()
			 *
			 * 用户发起请求后，走到调用链，在经过 handlerMapping 拿到 uri 对应的 method 后，会调用合适的 adapter 进行方法调用。
			 * 1、如果是 @Controller 注解类的方法，通过反射调用
			 * 2、如果是实现的接口，可以直接调用。
			 *
			 */
            Map<String, HandlerAdapter> matchingBeans =
                    BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerAdapter.class, true, false);
            if (!matchingBeans.isEmpty()) {
                this.handlerAdapters = new ArrayList<>(matchingBeans.values());
                // We keep HandlerAdapters in sorted order.
                AnnotationAwareOrderComparator.sort(this.handlerAdapters);
            }
        } else {
            try {
                HandlerAdapter ha = context.getBean(HANDLER_ADAPTER_BEAN_NAME, HandlerAdapter.class);
                this.handlerAdapters = Collections.singletonList(ha);
            } catch (NoSuchBeanDefinitionException ex) {
                // Ignore, we'll add a default HandlerAdapter later.
            }
        }
        
        // Ensure we have at least some HandlerAdapters, by registering
        // default HandlerAdapters if no other adapters are found.
        if (this.handlerAdapters == null) {
            this.handlerAdapters = getDefaultStrategies(context, HandlerAdapter.class);
            if (logger.isTraceEnabled()) {
                logger.trace("No HandlerAdapters declared for servlet '" + getServletName() +
                        "': using default strategies from DispatcherServlet.properties");
            }
        }
    }
    
    /**
     * Initialize the HandlerExceptionResolver used by this class.
     * <p>If no bean is defined with the given name in the BeanFactory for this namespace,
     * we default to no exception resolver.
     */
    private void initHandlerExceptionResolvers(ApplicationContext context) {
        this.handlerExceptionResolvers = null;
        
        if (this.detectAllHandlerExceptionResolvers) {
            // Find all HandlerExceptionResolvers in the ApplicationContext, including ancestor contexts.
            Map<String, HandlerExceptionResolver> matchingBeans = BeanFactoryUtils
                    .beansOfTypeIncludingAncestors(context, HandlerExceptionResolver.class, true, false);
            if (!matchingBeans.isEmpty()) {
                this.handlerExceptionResolvers = new ArrayList<>(matchingBeans.values());
                // We keep HandlerExceptionResolvers in sorted order.
                AnnotationAwareOrderComparator.sort(this.handlerExceptionResolvers);
            }
        } else {
            try {
                HandlerExceptionResolver her =
                        context.getBean(HANDLER_EXCEPTION_RESOLVER_BEAN_NAME, HandlerExceptionResolver.class);
                this.handlerExceptionResolvers = Collections.singletonList(her);
            } catch (NoSuchBeanDefinitionException ex) {
                // Ignore, no HandlerExceptionResolver is fine too.
            }
        }
        
        // Ensure we have at least some HandlerExceptionResolvers, by registering
        // default HandlerExceptionResolvers if no other resolvers are found.
        if (this.handlerExceptionResolvers == null) {
            this.handlerExceptionResolvers = getDefaultStrategies(context, HandlerExceptionResolver.class);
            if (logger.isTraceEnabled()) {
                logger.trace("No HandlerExceptionResolvers declared in servlet '" + getServletName() +
                        "': using default strategies from DispatcherServlet.properties");
            }
        }
    }
    
    /**
     * Initialize the RequestToViewNameTranslator used by this servlet instance.
     * <p>If no implementation is configured then we default to DefaultRequestToViewNameTranslator.
     */
    private void initRequestToViewNameTranslator(ApplicationContext context) {
        try {
            this.viewNameTranslator =
                    context.getBean(REQUEST_TO_VIEW_NAME_TRANSLATOR_BEAN_NAME, RequestToViewNameTranslator.class);
            if (logger.isTraceEnabled()) {
                logger.trace("Detected " + this.viewNameTranslator.getClass().getSimpleName());
            } else if (logger.isDebugEnabled()) {
                logger.debug("Detected " + this.viewNameTranslator);
            }
        } catch (NoSuchBeanDefinitionException ex) {
            // We need to use the default.
            this.viewNameTranslator = getDefaultStrategy(context, RequestToViewNameTranslator.class);
            if (logger.isTraceEnabled()) {
                logger.trace("No RequestToViewNameTranslator '" + REQUEST_TO_VIEW_NAME_TRANSLATOR_BEAN_NAME +
                        "': using default [" + this.viewNameTranslator.getClass().getSimpleName() + "]");
            }
        }
    }
    
    /**
     * Initialize the ViewResolvers used by this class.
     * <p>If no ViewResolver beans are defined in the BeanFactory for this
     * namespace, we default to InternalResourceViewResolver.
     */
    private void initViewResolvers(ApplicationContext context) {
        this.viewResolvers = null;
        
        if (this.detectAllViewResolvers) {
            // Find all ViewResolvers in the ApplicationContext, including ancestor contexts.
            Map<String, ViewResolver> matchingBeans =
                    BeanFactoryUtils.beansOfTypeIncludingAncestors(context, ViewResolver.class, true, false);
            if (!matchingBeans.isEmpty()) {
                this.viewResolvers = new ArrayList<>(matchingBeans.values());
                // We keep ViewResolvers in sorted order.
                AnnotationAwareOrderComparator.sort(this.viewResolvers);
            }
        } else {
            try {
                ViewResolver vr = context.getBean(VIEW_RESOLVER_BEAN_NAME, ViewResolver.class);
                this.viewResolvers = Collections.singletonList(vr);
            } catch (NoSuchBeanDefinitionException ex) {
                // Ignore, we'll add a default ViewResolver later.
            }
        }
        
        // Ensure we have at least one ViewResolver, by registering
        // a default ViewResolver if no other resolvers are found.
        if (this.viewResolvers == null) {
            this.viewResolvers = getDefaultStrategies(context, ViewResolver.class);
            if (logger.isTraceEnabled()) {
                logger.trace("No ViewResolvers declared for servlet '" + getServletName() +
                        "': using default strategies from DispatcherServlet.properties");
            }
        }
    }
    
    /**
     * Initialize the {@link FlashMapManager} used by this servlet instance.
     * <p>If no implementation is configured then we default to
     * {@code org.springframework.web.servlet.support.DefaultFlashMapManager}.
     */
    private void initFlashMapManager(ApplicationContext context) {
        try {
            this.flashMapManager = context.getBean(FLASH_MAP_MANAGER_BEAN_NAME, FlashMapManager.class);
            if (logger.isTraceEnabled()) {
                logger.trace("Detected " + this.flashMapManager.getClass().getSimpleName());
            } else if (logger.isDebugEnabled()) {
                logger.debug("Detected " + this.flashMapManager);
            }
        } catch (NoSuchBeanDefinitionException ex) {
            // We need to use the default.
            this.flashMapManager = getDefaultStrategy(context, FlashMapManager.class);
            if (logger.isTraceEnabled()) {
                logger.trace("No FlashMapManager '" + FLASH_MAP_MANAGER_BEAN_NAME +
                        "': using default [" + this.flashMapManager.getClass().getSimpleName() + "]");
            }
        }
    }
    
    /**
     * Return this servlet's ThemeSource, if any; else return {@code null}.
     * <p>Default is to return the WebApplicationContext as ThemeSource,
     * provided that it implements the ThemeSource interface.
     * @return the ThemeSource, if any
     * @see #getWebApplicationContext()
     */
    @Nullable
    public final ThemeSource getThemeSource() {
        return (getWebApplicationContext() instanceof ThemeSource ? (ThemeSource) getWebApplicationContext() : null);
    }
    
    /**
     * Obtain this servlet's MultipartResolver, if any.
     * @return the MultipartResolver used by this servlet, or {@code null} if none
     * (indicating that no multipart support is available)
     */
    @Nullable
    public final MultipartResolver getMultipartResolver() {
        return this.multipartResolver;
    }
    
    /**
     * Return the configured {@link HandlerMapping} beans that were detected by
     * type in the {@link WebApplicationContext} or initialized based on the
     * default set of strategies from {@literal DispatcherServlet.properties}.
     * <p><strong>Note:</strong> This method may return {@code null} if invoked
     * prior to {@link #onRefresh(ApplicationContext)}.
     * @return an immutable list with the configured mappings, or {@code null}
     * if not initialized yet
     * @since 5.0
     */
    @Nullable
    public final List<HandlerMapping> getHandlerMappings() {
        return (this.handlerMappings != null ? Collections.unmodifiableList(this.handlerMappings) : null);
    }
    
    /**
     * Return the default strategy object for the given strategy interface.
     * <p>The default implementation delegates to {@link #getDefaultStrategies},
     * expecting a single object in the list.
     * @param context the current WebApplicationContext
     * @param strategyInterface the strategy interface
     * @return the corresponding strategy object
     * @see #getDefaultStrategies
     */
    protected <T> T getDefaultStrategy(ApplicationContext context, Class<T> strategyInterface) {
        List<T> strategies = getDefaultStrategies(context, strategyInterface);
        if (strategies.size() != 1) {
            throw new BeanInitializationException(
                    "DispatcherServlet needs exactly 1 strategy for interface [" + strategyInterface.getName() + "]");
        }
        return strategies.get(0);
    }
    
    /**
     * Create a List of default strategy objects for the given strategy interface.
     * <p>The default implementation uses the "DispatcherServlet.properties" file (in the same
     * package as the DispatcherServlet class) to determine the class names. It instantiates
     * the strategy objects through the context's BeanFactory.
     * @param context the current WebApplicationContext
     * @param strategyInterface the strategy interface
     * @return the List of corresponding strategy objects
     */
    @SuppressWarnings("unchecked")
    protected <T> List<T> getDefaultStrategies(ApplicationContext context, Class<T> strategyInterface) {
        String key = strategyInterface.getName();
        String value = defaultStrategies.getProperty(key);
        if (value != null) {
            String[] classNames = StringUtils.commaDelimitedListToStringArray(value);
            List<T> strategies = new ArrayList<>(classNames.length);
            for (String className : classNames) {
                try {
                    Class<?> clazz = ClassUtils.forName(className, DispatcherServlet.class.getClassLoader());
                    // 关键方法
                    Object strategy = createDefaultStrategy(context, clazz);
                    strategies.add((T) strategy);
                } catch (ClassNotFoundException ex) {
                    throw new BeanInitializationException(
                            "Could not find DispatcherServlet's default strategy class [" + className +
                                    "] for interface [" + key + "]", ex);
                } catch (LinkageError err) {
                    throw new BeanInitializationException(
                            "Unresolvable class definition for DispatcherServlet's default strategy class [" +
                                    className + "] for interface [" + key + "]", err);
                }
            }
            return strategies;
        } else {
            return new LinkedList<>();
        }
    }
    
    /**
     * Create a default strategy.
     * <p>The default implementation uses
     * {@link org.springframework.beans.factory.config.AutowireCapableBeanFactory#createBean}.
     * @param context the current WebApplicationContext
     * @param clazz the strategy implementation class to instantiate
     * @return the fully configured strategy instance
     * @see org.springframework.context.ApplicationContext#getAutowireCapableBeanFactory()
     * @see org.springframework.beans.factory.config.AutowireCapableBeanFactory#createBean
     */
    protected Object createDefaultStrategy(ApplicationContext context, Class<?> clazz) {
        return context.getAutowireCapableBeanFactory().createBean(clazz);
    }
    
    
    /**
     * Exposes the DispatcherServlet-specific request attributes and delegates to {@link #doDispatch}
     * for the actual dispatching.
	 * 请求来了之后进入此方法
     */
    @Override
    protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
        logRequest(request);
        
        // Keep a snapshot of the request attributes in case of an include,
        // to be able to restore the original attributes after the include.
        Map<String, Object> attributesSnapshot = null;
        if (WebUtils.isIncludeRequest(request)) {
            attributesSnapshot = new HashMap<>();
            Enumeration<?> attrNames = request.getAttributeNames();
            while (attrNames.hasMoreElements()) {
                String attrName = (String) attrNames.nextElement();
                if (this.cleanupAfterInclude || attrName.startsWith(DEFAULT_STRATEGIES_PREFIX)) {
                    attributesSnapshot.put(attrName, request.getAttribute(attrName));
                }
            }
        }
        
        // Make framework objects available to handlers and view objects.
		/**
		 * 将 spring 上下文对象存入 request 的 attribute
		 */
        request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
        // 国际化支持解析器放入 request
        request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
        // 主题解析器
        request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
        // 主题对象
        request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());
        
        if (this.flashMapManager != null) {
            FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
            if (inputFlashMap != null) {
                request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
            }
            request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
            request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);
        }
        
        try {
            /**
             * 核心逻辑，处理请求
             */
            doDispatch(request, response);
        } finally {
            if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
                // Restore the original attribute snapshot, in case of an include.
                if (attributesSnapshot != null) {
                    restoreAttributesAfterInclude(request, attributesSnapshot);
                }
            }
        }
    }
    
    private void logRequest(HttpServletRequest request) {
        LogFormatUtils.traceDebug(logger, traceOn -> {
            String params;
            if (isEnableLoggingRequestDetails()) {
                params = request.getParameterMap().entrySet().stream()
                        .map(entry -> entry.getKey() + ":" + Arrays.toString(entry.getValue()))
                        .collect(Collectors.joining(", "));
            } else {
                params = (request.getParameterMap().isEmpty() ? "" : "masked");
            }
            
            String queryString = request.getQueryString();
            String queryClause = (StringUtils.hasLength(queryString) ? "?" + queryString : "");
            String dispatchType = (!request.getDispatcherType().equals(DispatcherType.REQUEST) ?
                    "\"" + request.getDispatcherType().name() + "\" dispatch for " : "");
            String message = (dispatchType + request.getMethod() + " \"" + getRequestUri(request) +
                    queryClause + "\", parameters={" + params + "}");
            
            if (traceOn) {
                List<String> values = Collections.list(request.getHeaderNames());
                String headers = values.size() > 0 ? "masked" : "";
                if (isEnableLoggingRequestDetails()) {
                    headers = values.stream().map(name -> name + ":" + Collections.list(request.getHeaders(name)))
                            .collect(Collectors.joining(", "));
                }
                return message + ", headers={" + headers + "} in DispatcherServlet '" + getServletName() + "'";
            } else {
                return message;
            }
        });
    }
    
    /**
     * Process the actual dispatching to the handler.
     * <p>The handler will be obtained by applying the servlet's HandlerMappings in order.
     * The HandlerAdapter will be obtained by querying the servlet's installed HandlerAdapters
     * to find the first that supports the handler class.
     * <p>All HTTP methods are handled by this method. It's up to HandlerAdapters or handlers
     * themselves to decide which methods are acceptable.
	 *
	 * 依据请求找到对应的 controller 中的方法
	 *
     * @param request current HTTP request
     * @param response current HTTP response
     * @throws Exception in case of any kind of processing failure
     */
    protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
        HttpServletRequest processedRequest = request;
		/**
		 * 声明一个处理器执行链，
		 */
		HandlerExecutionChain mappedHandler = null;
        boolean multipartRequestParsed = false;
        
        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
        
        try {
            ModelAndView mv = null;
            Exception dispatchException = null;
            
            try {
				/**
				 * 检查是否是文件上传请求，特殊处理，
				 * 判断 request 请求头是否有 multipart-form-data ，说明是文件上传请求
				 */
				processedRequest = checkMultipart(request);
				
				/**
				 * 判断是不是我们文件上传的请求 若是的话返回的processedRequest是MultipartHttpServletRequest
				 * 那么显然和原始的request对象 不是同一个对象,那么就multipartRequestParsed为true(表示文件上传下载)
				 */
                multipartRequestParsed = (processedRequest != request);
                
                // Determine handler for the current request.
                /**
                 * 1、从我们当前的请求中推断出我们的 HandlerExecuteChain 处理器执行链对象
				 * 重点！！
				 * 经过这一步，通过 uri 获取到一个对应的 controller 的 method 方法
				 * （或者是一个类对象，因为注册 controller 的方式有 3 种，但是我们最常用的是用 @RequestMapping 注册 uri）
				 * 同时在 处理链加入了 适配的拦截器。
                 */
                mappedHandler = getHandler(processedRequest);
                if (mappedHandler == null) {
                    // 没有找到，则往 response 写入错误信息
                    noHandlerFound(processedRequest, response);
                    return;
                }
                
                // Determine handler adapter for the current request.
                /**
                 * 2、获取对应的 handlerAdapter 适配器，最终目的是要调用被适配的 handler 方法
				 * （实际最后调用的是目标 target 的方法，这里注意适配器模式的概念）
				 * 当前类对象相当于是一个 笔记本，adapter 相当于是电源转换器，而最终我们要将电压 220v 转为 5v。
				 ******
				 * 这里通过得到的 handler 类型，找到对应的适配器，通过适配器，可以调用与当前类毫无关系的类的方法。
				 * 比如我们的 MyBeanNameController 实现了 Controller 接口，得到了 Handler 类是对应的这个实现类
				 * （本 demo 中就是 MyBeanNameController）
				 * HandlerAdapter = 通过这个类的实现得到 Adapter = SimpleControllerHandlerAdapter
				 * 最终在下面会调用 ha.handle 方法，实际走的是 SimpleControllerHandlerAdapter.handle
				 * 看代码：直接转为了 Controller，然后调用了她的 handleRequest，实际就是 MyBeanNameController#handleRequest
				 * return ((Controller) handler).handleRequest(request, response)
				 * 然后分析一下 @RequestMapping 的方式
				 * 我们得到的是一个 HandlerMethod，然后通过 getAdapter 找到了 AbstractHandlerMethodAdapter
				 * 调用  AbstractHandlerMethodAdapter#handle 我们分析下代码，发现这里是通过反射调用对应的方法。
				 * 走到 RequestMappingHandlerAdapter#handleInternal
				 * 因为 HandlerMethod 已经定位到了具体的 controller 的方法了。
				 * ---
				 * 通过上面的 2 个例子，我们应该可以有点意识到了这里用适配器的原因。我们看 dispatcher 这里的职责就是发现请求，找到对应的方法，然后调用
				 * 但是怎么调用，它不关心，交给适配器去做，不论什么样的请求处理方式，交给对应的适配器去处理。
				 * dispatcher 只需要调用适配器的统一方法 handle 就可以了。所以即便是来了新的 controller 的处理类，dispatcher 根本不需要修改
				 * 只要你有对应的适配器就可以拿来用。
                 */
                HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
                
                // Process last-modified header, if supported by the handler.
                // 处理 last-modified 情况
                String method = request.getMethod();
                boolean isGet = "GET".equals(method);
                if (isGet || "HEAD".equals(method)) {
                    long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
                    if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
                        return;
                    }
                }
                
                if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                    return;
                }
                
                // Actually invoke the handler.
                /**
                 * 3、调用 handle，执行请求
				 * 通过我们的适配器真正的调用我们的目标方法
				 * 因为 controller 的定义方式有多种，所以对应的方法调用方式需要做适配。
				 */
                mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
                
                if (asyncManager.isConcurrentHandlingStarted()) {
                    return;
                }
                
                // 如果函数调用没有返回视图，则使用默认的
                applyDefaultViewName(processedRequest, mv);
				/**
				 * 触发我们拦截器链的post方法
				 */
                mappedHandler.applyPostHandle(processedRequest, response, mv);
            } catch (Exception ex) {
                dispatchException = ex;
            } catch (Throwable err) {
                // As of 4.3, we're processing Errors thrown from handler methods as well,
                // making them available for @ExceptionHandler methods and other scenarios.
                dispatchException = new NestedServletException("Handler dispatch failed", err);
            }
            /**
             * 4、处理目标方法返回的结果,主要就是渲染视图
             */
            processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
        } catch (Exception ex) {
			/**
			 * 抛出异常:处理我们拦截器的 afterCompletion 方法
			 */
            triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
        } catch (Throwable err) {
			/**
			 * 抛出异常:处理我们拦截器的 afterCompletion 方法
			 */
            triggerAfterCompletion(processedRequest, response, mappedHandler,
                    new NestedServletException("Handler processing failed", err));
        } finally {
            if (asyncManager.isConcurrentHandlingStarted()) {
                // Instead of postHandle and afterCompletion
                if (mappedHandler != null) {
                    mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
                }
            } else {
                // Clean up any resources used by a multipart request.
				/**
				 * 清除文件上传时候生成的临时文件
				 */
                if (multipartRequestParsed) {
                    cleanupMultipart(processedRequest);
                }
            }
        }
    }
    
    /**
     * Do we need view name translation?
     */
    private void applyDefaultViewName(HttpServletRequest request, @Nullable ModelAndView mv) throws Exception {
        if (mv != null && !mv.hasView()) {
            String defaultViewName = getDefaultViewName(request);
            if (defaultViewName != null) {
                mv.setViewName(defaultViewName);
            }
        }
    }
    
    /**
     * Handle the result of handler selection and handler invocation, which is
     * either a ModelAndView or an Exception to be resolved to a ModelAndView.
     */
    private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
            @Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
            @Nullable Exception exception) throws Exception {
        
        boolean errorView = false;
        
        if (exception != null) {
            if (exception instanceof ModelAndViewDefiningException) {
                logger.debug("ModelAndViewDefiningException encountered", exception);
                mv = ((ModelAndViewDefiningException) exception).getModelAndView();
            } else {
                Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
                mv = processHandlerException(request, response, handler, exception);
                errorView = (mv != null);
            }
        }
        
        // Did the handler return a view to render?
        if (mv != null && !mv.wasCleared()) {
            render(mv, request, response);
            if (errorView) {
                WebUtils.clearErrorRequestAttributes(request);
            }
        } else {
            if (logger.isTraceEnabled()) {
                logger.trace("No view rendering, null ModelAndView returned.");
            }
        }
        
        if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
            // Concurrent handling started during a forward
            return;
        }
        
        if (mappedHandler != null) {
            // Exception (if any) is already handled..
            mappedHandler.triggerAfterCompletion(request, response, null);
        }
    }
    
    /**
     * Build a LocaleContext for the given request, exposing the request's primary locale as current locale.
     * <p>The default implementation uses the dispatcher's LocaleResolver to obtain the current locale,
     * which might change during a request.
     * @param request current HTTP request
     * @return the corresponding LocaleContext
     */
    @Override
    protected LocaleContext buildLocaleContext(final HttpServletRequest request) {
        LocaleResolver lr = this.localeResolver;
        if (lr instanceof LocaleContextResolver) {
            return ((LocaleContextResolver) lr).resolveLocaleContext(request);
        } else {
            return () -> (lr != null ? lr.resolveLocale(request) : request.getLocale());
        }
    }
    
    /**
     * Convert the request into a multipart request, and make multipart resolver available.
     * <p>If no multipart resolver is set, simply use the existing request.
     * @param request current HTTP request
     * @return the processed request (multipart wrapper if necessary)
     * @see MultipartResolver#resolveMultipart
     */
    protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {
        if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) {
            if (WebUtils.getNativeRequest(request, MultipartHttpServletRequest.class) != null) {
                if (request.getDispatcherType().equals(DispatcherType.REQUEST)) {
                    logger.trace("Request already resolved to MultipartHttpServletRequest, e.g. by MultipartFilter");
                }
            } else if (hasMultipartException(request)) {
                logger.debug("Multipart resolution previously failed for current request - " +
                        "skipping re-resolution for undisturbed error rendering");
            } else {
                try {
                    return this.multipartResolver.resolveMultipart(request);
                } catch (MultipartException ex) {
                    if (request.getAttribute(WebUtils.ERROR_EXCEPTION_ATTRIBUTE) != null) {
                        logger.debug("Multipart resolution failed for error dispatch", ex);
                        // Keep processing error dispatch with regular request handle below
                    } else {
                        throw ex;
                    }
                }
            }
        }
        // If not returned before: return original request.
        return request;
    }
    
    /**
     * Check "javax.servlet.error.exception" attribute for a multipart exception.
     */
    private boolean hasMultipartException(HttpServletRequest request) {
        Throwable error = (Throwable) request.getAttribute(WebUtils.ERROR_EXCEPTION_ATTRIBUTE);
        while (error != null) {
            if (error instanceof MultipartException) {
                return true;
            }
            error = error.getCause();
        }
        return false;
    }
    
    /**
     * Clean up any resources used by the given multipart request (if any).
     * @param request current HTTP request
     * @see MultipartResolver#cleanupMultipart
     */
    protected void cleanupMultipart(HttpServletRequest request) {
        if (this.multipartResolver != null) {
            MultipartHttpServletRequest multipartRequest =
                    WebUtils.getNativeRequest(request, MultipartHttpServletRequest.class);
            if (multipartRequest != null) {
                this.multipartResolver.cleanupMultipart(multipartRequest);
            }
        }
    }
    
    /**
     * Return the HandlerExecutionChain for this request.
     * <p>Tries all handler mappings in order.
	 *
	 * 根据我们的Request的请求来推断出我们的 处理器执行链对象
	 *
     * @param request current HTTP request
     * @return the HandlerExecutionChain, or {@code null} if no handler could be found
     */
    @Nullable
    protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
		/**
		 * 判断 springmvc 容器中配置的 handlerMappings ，
		 * handlerMappings 是在 springmvc 容器初始化时注册到容器的
		 * org.springframework.web.servlet.DispatcherServlet#initHandlerMappings
		 */
		if (this.handlerMappings != null) {
            /**
             * 遍历所有的 mapping，得到对应的处理类，
             */
            for (HandlerMapping mapping : this.handlerMappings) {
				/**
				 * 得到能够处理的 handler，看方法详情
				 * org.springframework.web.servlet.handler.AbstractHandlerMapping.getHandler
				 */
                HandlerExecutionChain handler = mapping.getHandler(request);
                if (handler != null) {
                    return handler;
                }
            }
        }
        return null;
    }
    
    /**
     * No handler found -> set appropriate HTTP response status.
     * @param request current HTTP request
     * @param response current HTTP response
     * @throws Exception if preparing the response failed
     */
    protected void noHandlerFound(HttpServletRequest request, HttpServletResponse response) throws Exception {
        if (pageNotFoundLogger.isWarnEnabled()) {
            pageNotFoundLogger.warn("No mapping for " + request.getMethod() + " " + getRequestUri(request));
        }
        if (this.throwExceptionIfNoHandlerFound) {
            throw new NoHandlerFoundException(request.getMethod(), getRequestUri(request),
                    new ServletServerHttpRequest(request).getHeaders());
        } else {
            response.sendError(HttpServletResponse.SC_NOT_FOUND);
        }
    }
    
    /**
     * Return the HandlerAdapter for this handler object.
     * @param handler the handler object to find an adapter for
     * @throws ServletException if no HandlerAdapter can be found for the handler. This is a fatal error.
     */
    protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
        if (this.handlerAdapters != null) {
            for (HandlerAdapter adapter : this.handlerAdapters) {
				/**
				 * 依据 Handler 的类型【
				 * HandlerMethod（AbstractHandlerMethodAdapter）、
				 * 自定义 MyBeanNameController 的类型（SimpleControllerHandlerAdapter）、
				 * 自定义 MyHttpRequestHandler 类（HttpRequestHandlerAdapter）
				 * 】 判断找到合适的 adapter
				 * 1、AbstractHandlerMethodAdapter 中会去判定是否是 HandlerMethod 对象，
				 * 【那么什么情况下这个 handler 是一个 HandlerMethod？使用注解注册我们的 uri 的时候，会追踪到具体的 method，
				 * 所以在 HandlerMapping 中存储的是 HandlerMethod 对象】
				 * 2、HttpRequestHandler 实现了HttpRequestHandler接口的类，那么就判定是否是HttpRequestHandler这个类型
				 * 因为这个 uri 对应的是一个类
				 * 3、SimpleControllerHandlerAdapter
				 * 4、SimpleServletHandlerAdapter 是一个 servlet
				 */
				if (adapter.supports(handler)) {
                    return adapter;
                }
            }
        }
        throw new ServletException("No adapter for handler [" + handler +
                "]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
    }
    
    /**
     * Determine an error ModelAndView via the registered HandlerExceptionResolvers.
     * @param request current HTTP request
     * @param response current HTTP response
     * @param handler the executed handler, or {@code null} if none chosen at the time of the exception
     * (for example, if multipart resolution failed)
     * @param ex the exception that got thrown during handler execution
     * @return a corresponding ModelAndView to forward to
     * @throws Exception if no error ModelAndView found
     */
    @Nullable
    protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,
            @Nullable Object handler, Exception ex) throws Exception {
        
        // Success and error responses may use different content types
        request.removeAttribute(HandlerMapping.PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE);
        
        // Check registered HandlerExceptionResolvers...
        ModelAndView exMv = null;
        if (this.handlerExceptionResolvers != null) {
            for (HandlerExceptionResolver resolver : this.handlerExceptionResolvers) {
                exMv = resolver.resolveException(request, response, handler, ex);
                if (exMv != null) {
                    break;
                }
            }
        }
        if (exMv != null) {
            if (exMv.isEmpty()) {
                request.setAttribute(EXCEPTION_ATTRIBUTE, ex);
                return null;
            }
            // We might still need view name translation for a plain error model...
            if (!exMv.hasView()) {
                String defaultViewName = getDefaultViewName(request);
                if (defaultViewName != null) {
                    exMv.setViewName(defaultViewName);
                }
            }
            if (logger.isTraceEnabled()) {
                logger.trace("Using resolved error view: " + exMv, ex);
            } else if (logger.isDebugEnabled()) {
                logger.debug("Using resolved error view: " + exMv);
            }
            WebUtils.exposeErrorRequestAttributes(request, ex, getServletName());
            return exMv;
        }
        
        throw ex;
    }
    
    /**
     * Render the given ModelAndView.
     * <p>This is the last stage in handling a request. It may involve resolving the view by name.
     * @param mv the ModelAndView to render
     * @param request current HTTP servlet request
     * @param response current HTTP servlet response
     * @throws ServletException if view is missing or cannot be resolved
     * @throws Exception if there's a problem rendering the view
     */
    protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
        // Determine locale for request and apply it to the response.
        Locale locale =
                (this.localeResolver != null ? this.localeResolver.resolveLocale(request) : request.getLocale());
        response.setLocale(locale);
        
        View view;
        String viewName = mv.getViewName();
        if (viewName != null) {
            // We need to resolve the view name.
            view = resolveViewName(viewName, mv.getModelInternal(), locale, request);
            if (view == null) {
                throw new ServletException("Could not resolve view with name '" + mv.getViewName() +
                        "' in servlet with name '" + getServletName() + "'");
            }
        } else {
            // No need to lookup: the ModelAndView object contains the actual View object.
            view = mv.getView();
            if (view == null) {
                throw new ServletException("ModelAndView [" + mv + "] neither contains a view name nor a " +
                        "View object in servlet with name '" + getServletName() + "'");
            }
        }
        
        // Delegate to the View object for rendering.
        if (logger.isTraceEnabled()) {
            logger.trace("Rendering view [" + view + "] ");
        }
        try {
            if (mv.getStatus() != null) {
                response.setStatus(mv.getStatus().value());
            }
            view.render(mv.getModelInternal(), request, response);
        } catch (Exception ex) {
            if (logger.isDebugEnabled()) {
                logger.debug("Error rendering view [" + view + "]", ex);
            }
            throw ex;
        }
    }
    
    /**
     * Translate the supplied request into a default view name.
     * @param request current HTTP servlet request
     * @return the view name (or {@code null} if no default found)
     * @throws Exception if view name translation failed
     */
    @Nullable
    protected String getDefaultViewName(HttpServletRequest request) throws Exception {
        return (this.viewNameTranslator != null ? this.viewNameTranslator.getViewName(request) : null);
    }
    
    /**
     * Resolve the given view name into a View object (to be rendered).
     * <p>The default implementations asks all ViewResolvers of this dispatcher.
     * Can be overridden for custom resolution strategies, potentially based on
     * specific model attributes or request parameters.
     * @param viewName the name of the view to resolve
     * @param model the model to be passed to the view
     * @param locale the current locale
     * @param request current HTTP servlet request
     * @return the View object, or {@code null} if none found
     * @throws Exception if the view cannot be resolved
     * (typically in case of problems creating an actual View object)
     * @see ViewResolver#resolveViewName
     */
    @Nullable
    protected View resolveViewName(String viewName, @Nullable Map<String, Object> model,
            Locale locale, HttpServletRequest request) throws Exception {
        
        if (this.viewResolvers != null) {
            for (ViewResolver viewResolver : this.viewResolvers) {
                View view = viewResolver.resolveViewName(viewName, locale);
                if (view != null) {
                    return view;
                }
            }
        }
        return null;
    }
    
    private void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response,
            @Nullable HandlerExecutionChain mappedHandler, Exception ex) throws Exception {
        
        if (mappedHandler != null) {
            mappedHandler.triggerAfterCompletion(request, response, ex);
        }
        throw ex;
    }
    
    /**
     * Restore the request attributes after an include.
     * @param request current HTTP request
     * @param attributesSnapshot the snapshot of the request attributes before the include
     */
    @SuppressWarnings("unchecked")
    private void restoreAttributesAfterInclude(HttpServletRequest request, Map<?, ?> attributesSnapshot) {
        // Need to copy into separate Collection here, to avoid side effects
        // on the Enumeration when removing attributes.
        Set<String> attrsToCheck = new HashSet<>();
        Enumeration<?> attrNames = request.getAttributeNames();
        while (attrNames.hasMoreElements()) {
            String attrName = (String) attrNames.nextElement();
            if (this.cleanupAfterInclude || attrName.startsWith(DEFAULT_STRATEGIES_PREFIX)) {
                attrsToCheck.add(attrName);
            }
        }
        
        // Add attributes that may have been removed
        attrsToCheck.addAll((Set<String>) attributesSnapshot.keySet());
        
        // Iterate over the attributes to check, restoring the original value
        // or removing the attribute, respectively, if appropriate.
        for (String attrName : attrsToCheck) {
            Object attrValue = attributesSnapshot.get(attrName);
            if (attrValue == null) {
                request.removeAttribute(attrName);
            } else if (attrValue != request.getAttribute(attrName)) {
                request.setAttribute(attrName, attrValue);
            }
        }
    }
    
    private static String getRequestUri(HttpServletRequest request) {
        String uri = (String) request.getAttribute(WebUtils.INCLUDE_REQUEST_URI_ATTRIBUTE);
        if (uri == null) {
            uri = request.getRequestURI();
        }
        return uri;
    }
    
}
